One of the primary mechanisms of gene regulation in prokaryotes involves the interaction between specific DNA sequences, called operators, and regulatory proteins, called repressors. In general a repressor will interact with only one operator, or a small set of operators. This binding interaction is very tight and specific for the operator sequence. When repressor is bound to the operator site, it prevents the interaction of RNA polymerase with its promoter site, a sequence of DNA which overlaps the operator and is the site at which RNA polymerase binds to initiate transcription.
Several prokaryotic regulatory systems have been studied in detail. These include the E. coli lac and trp operon regulatory systems, and the bacteriophage lambda major leftward and rightward promoters, pL and pR. [See, e.g., Beckwith, J., Escherichia coli and Salmonella typhimurium, Cellular and Molecular Biology, (F. Neidhardt, ed.) ASM Press, Washington, D.C., p1444-1452 (1987); Yanofsky, C. and Crawford, I., Escherichia coli and Salmonella typhimurium, Cellular and Molecular Biology, (F. Neidhardt, ed.) ASM Press, Washington, D.C., p1453-1472 (1987); and Gussin, G., et al., Lambda II (Hendrix, Roberts, Stahl, and Weisberg, eds) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., p93-121 (1983)]. Moreover these systems have been used to develop controllable expression vectors for the intracellular production of heterologous proteins [See, e.g., Remaut et al, Gene, 15:81 (1981); Rosenberg et al, Meth. Enzymol., 101:123 (1983); Backman and Ptashne (1978) Cell, 13:65 (1978); Edman et al, Nature, 291:503 (1981)]. All of these expression systems have focussed on using strong promoters so that transcription is maximal and expression rates are high.
Attempts have also been made to develop systems which will secrete heterologous proteins from bacterial cells. [See, e.g., U.S. Pat. No. 4,411,994; Masui et al, Bio/Technology, 81-85 (January 1984); and Emerick et al, Bio/Technology, 165-168 (February 1984)]. These systems have generally given disappointingly low yields of the desired protein. Once again relatively strong promoters have generally been employed to generate the message encoding the protein to be secreted.